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In CuSO4 solution, why doesn't Cu(OH)2 precipitate out?

  1. Dec 13, 2009 #1
    An aqueous solution of CuSO4 has a pH around 4. That means there are more H+ ions than OH-. Since the H+ ions come from dissociation of water, there must be an equal amount of OH-. Where have all the OH- ions gone? I suppose they are locked up as Cu(OH)2. However, the latter is highly insoluble in water. So why doesn't Cu(OH)2 precipitate out? I guess chelation of Cu(OH)2 by H3O+ may make it soluble. However, I have been unable to find any information about such chelation. Can anyone help?

    CuCl2, AlSO4, CaCl2 etc. all pose a similar problem.

    Many TIAs.

    Wai Wong
     
  2. jcsd
  3. Dec 13, 2009 #2

    Borek

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    Staff: Mentor

    Solution may contain some amount of copper (or other cation) complexed by OH- groups. There usually whole families of soluble complexes - like CuOH+, Cu2(OH)22+, Cu(OH)42- - involved.

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  4. Dec 13, 2009 #3
    Thanks Borek!

    BTW, I think Cu(OH)42- shouldn't be there, or otherwise Cu(OH)2 would be soluble in water or alkaline solutions.

    Wai Wong
     
  5. Dec 14, 2009 #4

    Borek

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    Staff: Mentor

    It depends on the relative values of the stability constant and solubility product. These are taken from the equilibria database, that means it is possible to trace the source of the information.

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  6. Dec 14, 2009 #5
    I am not good at finding those constants, but according to several sources, Cu(OH)42- forms only at very high pH, suggesting a low stability constant.

    Never mind. The root of my question has already been well answered.

    Thanks

    Wai Wong
     
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